The tank has a short piece of filler tube comming from the tank. The you have a short piece of hose that is clamped around the filler comming from the tank and the filler that protrudes through the side of your cab. Move your seat forward and use a screw driver loosen the upper clamp and slowly pull the upper filler tube from the cab. Sometimes you will need to grease it or lubricate it well. Carefully use a utility knife to cut the old filler tube grommet so you can remove it.
After it's gone clean up the hole.You might want to prime it to protect it some more. Fit your new grommet in place, make sure you set it the correct way. Lubricate the inside with a non-attacing oil (cooking oil) and carefully push/slide the old filler neck on place (if it's a good grommet this will need a bit of muscle it's supposed to be tight), and re tighten clamp.
That's all there is to it.
Jake
